Heavy particle linear accelerator with continuous variation of output energy



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.9 *c: g g 5 LL L3 fi a g \9 O 8 an INVENTORZ H-ZEBUl/TET ATZDRNE United States Patent 3,332,024 HEAVY PARTICLE LINEAR ACCELERATOR WITH CONTINUOUS VARIATION OF OUT- PUT ENERGY Hubert Leboutet, Paris, France, assignor to CSF-Compagnie Generale tie Telegraphic Sans Fil, Paris, France Filed July 12, 1963, Ser. No. 294,687 Claims priority, application France, Sept. 4, 1962, 908,513, Patent 1,340,271 14 Claims. (Cl. 328-233) The present invention relates to linear ion accelerators, and more particularly to the linear ion accelerators of the type with travelling waves in which the particles to be accelerated move parallelly to a delay line and in synchronism with the phase velocity of the wave which propagates within this line, which line has a variable pitch to the extent the velocity of the particles has not attained the relativistic domain.

The aim of the present invention is an accelerator of this type which permits a continuous variation, within important limits, of the output energy of the accelerated rons.

The problem of output energy variation is simple and easy to resolve within linear accelerators of electrons, particles of a mass which is extremely slight and which are capable of a velocity which attains the relativistic domain very rapidly, before the energy reaches the order of magnitude practicable today. It sufiices then to vary the high frequency power injected into the delay circuit at the place where the pitch thereof becomes constant, the velocity of the electrons having attained a value near that of light and practically no longer varying as the energy increases. The output energy of the electrons their follows these variations of high frequency power continuously without any difliculty for the operation of the accelerator.

However, it is different when the accelerated particles are ions, that is, particles having a mass which is several thousand or several tens of thousands times greater than that of the electrons. With the orders of magnitude of output energy practicable today, the velocity of the particles at the output never attains the relativistic do main, and the interaction with the high frequency wave takes place along a circuit whose pitch is variable over the entire length of the accelerator. If in that case one arbitrarily assumes any law of variation of this pitch, and if one attempts to vary the output energy by acting either on the power or on the frequency of the injected wave, one only obtains variations of energy within extremely narrow limits. In effect, an accelerator having a variable pitch circuit following an arbitrary law operates only for a predetermined value of power and injected frequency, or within the immediate neighborhoods of this value. Each important deviation from this value has the result that the energy variation of the particles along the circuit ceases to follow continuously the variations of phase velocity of the wave, or that the particles are not, from the beginning, in synchronism with the wave. The accelerator then no longer operates or functions properly outside of the aforementioned narrow band.

According to the present invention, one realizes a linear ion accelerator with continuously variable energy within important limits by providing the accelerator with a delay line having a phase shift that is constant per cell and with a variable pitch, the law of variation of the pitch being chosen in a parti ular manner such that the accelerator equipped with a given line operates to accelerate ions while a simultaneously suitably combined combine-d regulation of the power and of the frequency of the injected wave determines the level of output energy.

This law for the variation of the pitch is particularly such that the pitch p, within each successive cell of the line, follows approximately the following law:

P= z in which 2 is the running coordinate, along the line, between a predetermined homologous point of each cell and a common predetermined point situated in front of the line; and

A is a coefiicient dependent on the mass of accelerated ions, of the injected high frequency power, and of the frequently utilized.

Additionally, the initial velocity of the particles will be such that the line operates on a predetermined d-ispersive space harmonic.

Accordingly, it is an object of the present invention to provide a heavy-particle linear accelerator, and particularly an ion accelerator which obviates the shortcomings and drawbacks encountered with the prior art constructions by simple means.

It is another object of the present invention to provide a linear accelerator for relatively heavier particles, such as ions, which permits, by simple means, the continuous variation of the output energy of the accelerated particles within a band that is relatively wide.

Still another object of the present invention resides in the provision of linear accelerators of the type mentioned hereinabove and operable to accelerate particles of such mass that the particles never attain the relativistic domain, which accelerator permits a continuous control with simple means to change the output energy of the accelerated particles by varying the power of the injected wave and/or the frequency thereof.

A further object of the present invention resides in the provision of a delay line for use in linear particle accelerators which is so constructed and arranged that the high frequency field effectively traverses within each cell of the line a constant distance, thereby permitting a continuous variation of the energy output of the accelerated particles.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, several embodiments in accordance with the present invention and wherein:

FIGURE 1 is a schematic diagram explanatory of the involved principles of dimension of the delay lines according to the present invention,

FIGURE 2 is a diagram of distribution of velocity of energy variation along the line in accordance with the present invention,

FIGURE 3 is a partial elevational view of a delay line constructed in accordance with the present invention, and

FIGURE 4 is a modified embodiment of a delay line constructed and dimensioned in accordance with the present invention and incorporated into an ion accelerator.

Referring now to the drawing and more particularly to FIGURE 1, this figure illustrates a chain of cells with variable pitch p, taking in the successive cells the values p p p and it being assumed that the variable pitch is such that it is possible to define within each cell the paths effectively traversed by the high frequency field, this path A, B, C, D, being of meander shape and its length per cell being I. Numerous lines such as meander shaped, interdigital, bent coaxial lines and others are representative examples of such a chain of cells.

3 For such a chain, the phase shift per cell may be expressed as follows:

col 9 r// c +J01r in which is the velocity of light,

w=21rf= pulsation of the frequency f, and

k=an entire number that may be positive, negative or zero.

It may be seen from the foregoing that to cause =constant for a given frequency, it suffices to make l constant for all the cells.

The first characteristic of the lines utilizable according to the present invention is, therefore, that one dimensions the lines in such a manner that the high frequency field travels effectively within each cell a constant distance.

On the other hand, the apparent phase velocity of the wave in the axial direction of the line may be expressed as follows:

By combining the Equations 1 and 2, one obtains the dispersion equation of the line which is as follows:

Ike

It may be seen from the foregoing that in the fundamental space mode (k-=0) such a line is not dispersive. In contrast, the line is dispersive in the harmonic modes (k O), whereby its utilization on the fundamental mode or on a predetermined harmonic depends exclusively on the initial velocity of the particles injected into the accelerator,

The second characteristic of the lines utilizable according to the present invention is, therefore, that they be utilized on a dispersive harmonic.

It is thereupon assumed that one ning of the line a high frequency energy eV (e being the charge of the particles and V the voltage related by the expression V /2PZ to theinjected power P and a quantity Z which depends only on the geometry and the electrical qualities of the line), and that the particles arrive at the beginning of the delay line with an initial energy injects at the begin- I I0 *(Fhii and an initial coordinate z representing the distance between a predetermined point of the first cell, for example, between its beginning A and a point 0 located ahead of the line, which becomes a predetermined point predetermined by this distance Z11 l t 3 Vu( f1 (5) It must be noted that with p and Z one may define the quantity as follows:

P /a 1 s(l 1) A 1 3 0 m 'i (6) Having thus determined the values of p; and z one proceeds to the determination of the pitchesof the following successive cells, =by proceeding, step by step, with 4 the utilization of the following relationship, constituting the third characteristic of the lines utilizable according to the present invention:

The characteristics and dimension of the delay line having thus completely been determined, one will now justify that the accelerator equipped with such a line satisfies the proposed aim, that is that it is operable to,

supply ions with variable energy within wide limits;

In the first place, according to Equation3 one has:

This relation providing the phase velocity for eachcell of the line, therefore also for the first cell as follows:

On the other hand, according to Equation 4 the initial velocity of the particle is as follows:

particles are correctly injected in synchronism with the phase of the wave.

The condition necessary for proper operation of the accelerator is that the velocity of the particles follows as continuously as possible the variations of the phase velocity of the wave, and it will be shown hereinafter that this is the case at the sole condition of the maintenance at a constant value of the quantity A defined by Equation 6.,

To make this demonstration, it will be noted that, the values ofthe pitches p being small in relation to z;, it is not necessary to take as homologous point of each cell the beginning thereof as has been made in the embodiment of FIGURE 1, but one could also define the coordinates Z Z z asgoing from the point 0 to any homologous point of each cell, for example, to its center point or terminal point. At the limit, one may replace with an admissible approximation the discontinuous law defined by Equation 7, by the law of variation of the pitch along the running coordinate 2 as follows:

P= in which'the quantity A is that defined by Equation 6.

The Equation 9, then provides the law of variation of the phase velocity along the circuit as follows:

If the velocity of the particles follows continuously this phase velocity, the energy of the particles Searching for the velocity of variation of this energy along the line, one obtains the following equation:

The equation 15 must be interpreted by taking duly into consideration the approximation admitted hereinabove, for, as shows FIGURE 2 of the drawing, in which has been represented graphically the two terms of this equation, the function eV /p is a discontinuous curve in steps whereas the function aW/az is a continuous curve. The exact interpretation of this equation is therefore that within the lines dimensioned and utilized according to the given indications, the velocity of variation of the energy along the circuit coincides with the continuous curve average of the discontinuous curve representing eV /p.

Onthe other hand, the quantity eV /p being constant on the inside of each .cell, it is, in the n cell, equal to eV /p and the absolute increase of the energy in the n' cell is:

In the lines according to the present invention, the energy therefore increases by a constant quantity eV in each cell, independently of the frequency or the mass of the quantity.

However, it is known, as for example indicated by the Equation 28 in the article by Alvarez et a1. entitled, Berkeley Proton Linear Accelerator, the Review of Scientific Instruments, vol. 26, No. 2, February 1955, page 120, that when this increase of energy is constant, the phase of the wave as seen from the particle which presents itself in each cell is always the same. The particle is then so-called synchronous, and its velocity follows continuously the variations of the phase velocity of the wave, which is an essential condition in order that the accelerator be operative, condition admitted in the preceding analysis and now demonstrated a posteriori.

Having demonstrated that the accelerator is operative, one will now have recourse to its new possibilities insofar as the variation of the energy of the accelerated ions is concerned.

As shown by Equation 12, the line constructed according to the principles indicated hereinabove remains utilizable as long as the coefiicient A remains constant.

According to the Equation 6, this condition is subordinated to the constancy of the quantity:

6V l k 2 A -+f) constant It may therefore be seen that for a given mass m it is possible to cause simultaneously the injected power represented by V and the frequencies f to vary within the limits of the band-pass of the line, in such a manner, as to maintain the Expression 16 at the same value as that which corresponded to the frequency f taken as starting point of the calculation.

On the other hand, the same line is utilizable for the ions of different chemical elements, whereby the variation of the mass m may then be compensated by the variation of one of the parameters V or f or of the two together in such a manner as to maintain the quantity (16) at the same value as with the ions taken as starting point.

However, as shown by Equation 14 the energy of the ions, with A constant, varies with V such that the action on V produces the desired energy variation within the limits which are determined only by the band-pass beyond which the frequency j that one causes to vary simultaneously with V must not pass that is, within relatively wide limits.

FIGURE 3 shows one embodiment of a delay line according to the present invention, in the form of a coaxial line folded or bent in meander shape, comprising an internal conductor 1 and an external conductor 2. According to the given indications, the folding is made in such a manner that the length of the arcs l remains constant whereas the pitch p 11 p etc. are dimensioned according to the Equations 4 and 7.

FIGURE 4 illustrates another embodiment of a line, incorporated into an ion accelerator of which the essential elements are only illustrated schematically since they may be of any conventional construction. This line comprises, within the axis of a parallelepipedic'cavity 3, a series of drift tubes 4, supported by rods 5 secured alternately to the opposite walls of the cavity 3, in the manner of an interdigital line. Between the tubes 4 passesin slalom fashion a conductor 6 bent in meander shape and having its extremities fixed to the end walls of the cavity 3. The dimensions of the arcs l and of the pitches p p p is the same as in FIGURE 3 and conforms to the given indications pointed out hereinabove. The conductor 6, at the places of intersection with the axis of the drift tubes 4, is pierced by orifices 7 for the passage of the beam 11, which enters and leaves the cavity by orifices 8 and 9, provided within the end walls thereof. Such a delay circuit, utilizing drift tubes 4 and a meander-shaped conductor 6, forms part of the copend-ing application Ser. No. 291,337, filed on June 28, 1963, and entitled, Delay Circuit Structure, filed in the name of Hubert Leboutet and Germaine Vincent, assigned to the assignee of the present application, however the particular dimensioning of such delay line in accordance with the present invention is novel to the invention herein.

This delay line forms part of the ion accelerator, the ions being supplied by a source 10 of which the ions are concentrated into a beam 11 which is focused by the windings 12. After having traversed the cavity 3 through the orifices 8 and 9, the accelerated beam enters into the utilization chamber 13 containing, for example, a target 14. The vacuum is maintained within the cavity 3 by a pump 15.

The high frequency power is injected into the delay line by means of a generator 16 coupled to the cavity 3 by a loop 17. The power and the frequency of this generator are adjustable by any conventional known means, not shown in detail herein, and indicated only schematically by the control buttons 18 and 19. These regulations and adjustments are eifectuated according to the indications given hereinabove, as a function of the mass of the ions supplied by the source 10 and the desired energy on the target 14. Additionally, a control member 20 is provided in the source 10 to impart to the particles a velocity such that the interaction takes place with a desired dispersive harmonic mode of the high frequency wave propagating within the line.

While I have shown and described two embodiments in accordance with the present invention, it is understood that the same is not limited to the examples described and illustrated herein, but encompasses all delay circuits dimensioned according to the principles indicated herein for utilization in the ion accelerators, whereby these circuits may be of the general type in which the wave travels along a meander or a helix about a longitudinal axis corresponding to the movement of the accelerated beam.

Thus, while I have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and I therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

I claim:

1. A linear ion accelerator, comprising a substantially linearly extending delay circuit, a source of ions provided with means for directing a beam of said ions along the linear extension of said delay circuit, means operable to excite in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic, said circuit being formed ofa chain of cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the efiective propagation path of said Wave in each cell, and the size of said cells along-said linear extension increasing substantially in proportion to the cube root of the distance of each 1 respective cell along said linear extension to a predetermined point ahead of said circuit.

2. A linear ion accelerator, comprising a substantially linearly extending delay circuit, a source of ions provided with means for directing a beam of said ions along the linear extension of said delay circuit, means operable to excite in said circuita travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic, said circuit being formed of a chain of cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the effective propagation path of said wave in each cell, and the'size of said cells along said linear extension increasing substantially in proportion to the cube root of the distance of each respective cell along said linear extension of a predetermined point ahead of said circuit, the dispersive space harmonic being a harmonic of a rank other than the fundamental.

3. A linear ion accelerator, comprising a substantially linearly extending delay circuit, a source of ions provided.

with means for directing a beam of said ions along the linear extension of said delay circuit, means operable to excite in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting said beam velocity to be synchronous withthe phase velocity of a predetermined dispersive space harmonic, said circuit'being formed of a chain of cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the efiective propagation path of said Wave in each cell, and the size of said cells along said linear extension increasing substantially in proportion to cube root of the distance of each respective cell along said linear extension to a predetermined point ahead of said circuit, the said propagation path being arranged to oscillate about an axis which extends substantially in the sense of said linear extension to form arches along said path, the length of each arch of said path between two successive points of intersection thereof with said axis being substantially constant, and the amplitude of said oscillation decreasing successively for each of said arches in the direction of said beam.

4. A linear ion accelerator, comprising a substantially linearly extending delay circuit, a source of ions provided wtih means for directing a beam of said ions along the linear extension of said delay circuit, means operable to excite in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic, said circuit being formed of a chain cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the effective propagation path of said wave in each cell, and the size of said cells along said linear extension increasing substantially in proportion to the cube root of the distance of each respective cell along said linear extension to a predetermined point ahead of said circuit, and the distance Z between the first of said cells andsaid predetermined point being given by the equation l '1 3 0 f1 A wherein m is the mass of ion, eV is the high frequency power of the exciting means, I is the length of the effective propagation path in each cell, c is the velocity of light, k is an integral number other than zero expressing the rank of the dispersive harmonic, f is a predetermined frequency of the exciting means selected within the passband of the circuit, and v is the initial velocity of the ion beam.

5. An accelerator as claimed in claim 4, wherein the linearly extending dimension p of said first cell is given by the equation:

I k Pro 6. An accelerator as claimed in claim 4,wherein successive linearly extending dimensions of respectlve cells,

with n taking succesively the values 1, 2, 3, etc. and Where Z2, Z Z4, z z are the successive distances between each of the respective cells and said pre wherein z is the distance between a given cell and said predetermined point, and A is given by,

8. A linear ion accelerator, comprising a substantially linearly extending delay circuit, a source of ions provided with means for directing a beam of said ionsalong the linear extension of said delay circuit, means operable to excite in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said'beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined, dispersive space harmonic, said circuit being formed of a chain of cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the effective propagation path of-said wave in each cell, and the size of said cells along said linear extension increasing substantially in pro portion to the cube root of the distance of each respective cell along said linear extension to a predetermined point ahead of said circuit, said delay circuit including a coaxial line undulatorily bent to oscillate about an axis extending substantially in the direction of said linear extension and thereby form arched portions, thelength of each of said arched portions between two successive points ofintersection with said axis being substantially constant, and the amplitude of said oscillation decreasing successively for each of said arched portions in the direction of said beam.

9. A linear ion accelerator, comprising a substantially linearily delay circuit, a source of ions provided with means for directing a beam of said ions along the linear extension of said delay circuit, means operable to excite in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic, said circuit being formed of a' chain of cascade coupled cells, said cells beingdimensionedto ensure a substantially are determined by the constant length of the effective propogation path of said wave in each cell, and the size of said cells along said linear extension increasing substantially in proportion to the cube root of the distance of each respective cell along said linear extension to a predetermined point ahead of said circuit, first adjusting means for adjusting the power of said exciting means and second adjusting means for adjusting the frequency of said traveling wave thereby adjusting over a relatively wide band the output of said ion beam.

10. In a linear ion accelerator having a substantially linearly extending delay circuit portion, a source of ions provided with means for directing a beam of said ions along said linearly extending portion of said delay circuit, means for exciting in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting the beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic,

the improvement essentially consisting of means in said line to enable adjustment of the energy output of the ion beam over a relatively wide band by selectively varying at least one of the two parameters consisting of the frequency of the travelling wave and the power input of said exciting means.

11. A linear ion accelerator, comprising a substantially linearly extending delay circuit, a source of ions provided with means for directing a beam of said ions along the linear extension of said delay circuit, means operable to excite in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic, said circuit being formed of a chain of cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the efiective propagation path of said wave in each cell, and the size of said cells along said linear extension increasing substantially in proportion to the cube root of the distance of each respective cell along said linear extension to a predetermined point ahead of said circuit, the dispersive space harmonic being a harmonic of a rank other than the fundamental, first adjusting means for adjusting the power of said exciting means and second adjusting means for adjusting the frequency of said travelling wave thereby adjusting over a relatively wide band the output power of said ion beam.

12. A linear ion accelerator, comprising a substantially linearly extending delay circuit portion, a source of ions provided with means for directing a beam of said ions along said delay circuit portion, means for exciting in said circuit portion a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction a said beam, and means for adjusting said beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic, said circuit portion being formed of a chain of cascade coupled cells, said cells being dimensioned to ensure a substantially constant length of the effective propagation path of said wave in each cell, and the size of said cells along said circuit portion increasing substantially in proportion to the cube root of the distance of each respective cell along said circuit portion with respect to a predetermined point ahead of said circuit, said circuit portion including a cavity resonator bounded by an envelope having at least one axis of symmetry, a series of drift tubes located along said axis, each of said tubes being supported 'by a rod-shaped element, said rodshaped elements being situated in a common plane passing through said axis and being alternately fixed to opposite points of said envelope thereby forming an interdigital structure in an axial plane of said envelope, and a conductor undulating to and fro between said successive tubes in a plane passing through said axis and having its extremities fixed to said envelope, the length of each arch of said undulation between two successive points of intersection with said axis being substantially constant, the amplitude of said undulation decreasing and the pitch of said undulation increasing successively for each of said arches in the direction of said beam, said increase being substantially in proportion to the cubic root of the distance of each respective of said intersection points with respect to a predetermined point on said axis ahead of said circuit.

13. In a linear ion accelerator having a substantially linearly extending delay circuit portion having a plurality of cells, a source of ions provided with means for directing a beam of said ions along said linearly extending portion of said delay circuit, means for exciting in said circuit a travelling wave having at least one space harmonic component propagating with a phase velocity in the same direction as said beam, and means for adjusting the beam velocity to be synchronous with the phase velocity of a predetermined dispersive space harmonic,

the improvement essentially consisting of means in said line to enable adjustment of the energy output of the ion beam over a relatively wide band by selectively varying at least one of the two parameters consisting of the frequency of the travelling wave and the power input of said exciting means including means providing a substantially constant length of the effective propagation path for the wave in each cell, and the dimension of said cells along said linearly extending portion increasing approximately in proportion to the cube root of the distance of each respective cell along said linearly extending portion to a predetermined point ahead of said circuit.

14. A delay circuit comprising: a cavity resonator bounded by an envelope having at least one axis of symmetry, a series of drift tubes located along said axis, each of said tubes being supported by a rod-shaped element, said rod-shaped elements being situated in a common plane passing through said axis and being alternately fixed to opposite points of said envelope thereby forming an interdigital structure in an axial plane of said envelope, and a conductor undultaing to and fro between said successive tubes in a plane passing through said axis and having its extremities fixed to said envelope, the length of each arch of said undulation between two successive points of intersection with said axis being substantially constant, the amplitude of said undulation decreasing and the pitch of said undulation increasing successively for each of said arches in the direction of said beam, said increase being substantially in proportion to the cube root of the distance of each respective of said intersection points with respect to a predetermined point on said axis ahead of said circuit.

References Cited UNITED STATES PATENTS 2,770,755 11/1956 Good 3l5-5 2,836,759 5/1958 Colgate 3155.41 3,067,359 12/1962 Pottier 3155.42 3,147,396 9/1964 Goerz et all 315--5.42

HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, Assistant Examiner. 

1. A LINEAR ION ACCELERATOR, COMPRISING A SUBSTANTIALLY LINEARLY EXTENDING DELAY CIRCUIT, A SOURCE OF IONS PROVIDED WITH MEANS FOR DIRECTING A BEAM OF SAID IONS ALONG THE LINEAR EXTENSION OF SAID DELAY CIRCUIT, MEANS OPERABLE TO EXCITE IN SAID CIRCUIT A TRAVELLING WAVE HAVING AT LEAST ONE SPACE HARMONIC COMPONENT PROPAGATING WITH A PHASE VELOCITY IN THE SAME DIRECTION AS SAID BEAM, AND MEANS FOR ADJUSTING SAID BEAM VELOCITY TO BE SYNCHRONOUS WITH THE PHASE VELOCITY OF A PREDETERMINED DISPERSIVE SPACE HARMONIC, SAID CIRCUIT BEING FORMED OF A CHAIN OF CASCADE COUPLED CELLS, SAID CELLS BEING DIMENSIONED TO ENSURE A SUBSTANTIALLY CONSTANT LENGTH OF THE EFFECTIVE PROPAGATION PATH OF SAID WAVE IN EACH CELL, AND THE SIZE OF SAID CELLS ALONG SAID LINEAR EXTENSION INCREASING SUBSTANTIALLY IN PROPORTION TO THE CUBE ROOT OF THE DISTANCE OF EACH RESPECTIVE CELL ALONG SAID LINEAR EXTENSION TO A PREDETERMINED POINT AHEAD OF SAID CIRCUIT. 